1. Field of the Invention
The present invention relates to a lens barrel and a photographing apparatus incorporating the lens barrel. For example, the present invention relates to a lens barrel configured to drive an optical element, such as a focusing lens or a zoom lens, with a linear actuator along an optical axis, and to a photographing apparatus incorporating such a lens barrel. In particular, the present invention relates to an interchangeable lens barrel having an imaging optical system configured to bayonet-couple to a camera body having an image capture unit via a coupling mount, and to a photographing apparatus incorporating the interchangeable lens barrel.
2. Description of the Related Art
Heretofore, a variety of zoom lenses for video cameras or lenses for digital still cameras have been developed. Among them, a typical zoom lens for video cameras includes four lens units, i.e., in order from the object side, a fixed front lens unit, a variator lens unit movable along an optical axis for varying magnification, a fixed afocal lens unit, and a focusing lens unit movable along the optical axis for focusing and for maintaining a focal plane at a given location during the variation of magnification. Japanese Laid-Open Patent Application No. 2002-214504 discloses a driving device having a linear motor adapted to drive the focusing lens unit of such a zoom lens.
FIGS. 6A to 6D illustrate the structure of a driving device for driving a focusing lens unit with a linear motor as disclosed in the above Japanese Laid-Open Patent Application No. 2002-214504. FIG. 6A is a longitudinal sectional view showing the structure of a lens barrel containing a typical four-unit zoom lens. FIG. 6B is a sectional view taken along line 6B—6B of FIG. 6A.
In the lens barrel shown in FIGS. 6A and 6B, the zoom lens includes a front lens unit 501a, a variator lens unit 501b, an afocal lens unit 501c, and a focusing lens unit 501d. Guide bars 503, 504a, and 504b are disposed in parallel to an optical axis 505 to guide movable lens units, i.e., the variator lens unit 501b and the focusing lens unit 501d, and to prevent rotation thereof. A DC motor 506 serves as a drive source for moving the variator lens unit 501b. A front-lens tube 502 holds the front lens unit 501a. A variator (V) moving ring 511 holds the variator lens unit 501b. An intermediate frame 515 holds the afocal lens unit 501c. A rear-relay (RR) moving ring 514 holds the focusing lens unit 501d. 
The front-lens tube 502 is fixed in position to a rear tube 516. The guide bar 503 is positioned and supported by the front-lens tube 502 and the rear tube 516. A guide screw shaft 508 is also supported for rotation by the front-lens tube 502 and the rear tube 516. The guide screw shaft 508 is configured to be rotated by the rotation of an output shaft 506a of the DC motor 506 being transmitted via a gear train 507.
The V moving ring 511, which holds the variator lens unit 501b, includes a pressure spring 509 and a ball 510. The pressure spring 509 generates a spring force to cause the ball 510 to engage with a screw groove 508a formed on the guide screw shaft 508. When the guide screw shaft 508 is rotated by the DC motor 506, the V moving ring 511 moves forward or backward along the optical axis 505 while being guided and prevented from rotating by the guide bar 503.
The guide bars 504a and 504b are fitted and supported between the rear tube 516 and the intermediate frame 515. The intermediate frame 515 is fixed in position to the rear tube 516. The RR moving ring 514 is movable forward or backward along the optical axis 505 while being guided and prevented from rotating by the guide bars 504a and 504b. 
The RR moving ring 514, which holds the focusing lens unit 501d, has sleeve portions in which the guide bars 504a and 504b are slidably fitted. A rack 513 is mounted on the RR moving ring 514 such that the rack 513 is movable along the optical axis 505 integrally with the RR moving ring 514. A stepping motor 512 has a lead screw shaft 512a formed integrally with an output shaft thereof. When the stepping motor 512 is driven, the lead screw shaft 512a rotates. The rack 513, which is mounted on the RR moving ring 514, engages with the lead screw shaft 512a. When the lead screw shaft 512a rotates, the RR moving ring 514 moves along the optical axis 505 while being guided by the guide bars 504a and 504b. 
Similar to the drive source for the focusing lens unit 501d, a stepping motor may be used as a drive source for the variator lens unit 501b. 
The front-lens tube 502, the intermediate frame 515, and the rear tube 516 constitute a lens barrel body which contains the lens units 501a to 501d, etc., in an approximately hermetically-sealed manner.
Incidentally, in cases where a lens-unit holding frame is moved with such a stepping motor, a photointerrupter or the like is used to detect that the lens-unit holding frame is located in a reference position with respect to the optical axis direction. Then, the absolute position of the lens-unit holding frame is detected by continuously counting the number of driving pulses applied to the stepping motor.
A diaphragm unit 535 is configured to adjust the amount of incident light by driving a diaphragm mechanism disposed between the V moving ring 511 and the intermediate frame 515.
In the configuration shown in FIGS. 6A and 6B, the DC motor or the stepping motor is used to move the lens unit 501b or 501d. However, a linear actuator, such as a linear motor or a VCM (voice coil motor), can also be used for that purpose. FIG. 6C is a front elevation view as viewed from the optical axis direction of a portion around the focusing lens unit 501d shown in FIGS. 6A and 6B in an example in which a linear motor is used as a drive source for the focusing lens unit 501d. FIG. 6D is a perspective view of the linear motor shown in FIG. 6C.
In general, there are two types of the linear motor, a moving coil type and a moving magnet type. In the moving coil type, a coil is disposed on the movable side. In the moving magnet type, a magnet is disposed on the movable side. In the example shown in FIGS. 6C and 6D, the moving coil type is used.
Referring to FIGS. 6C and 6D, a coil 601 is integrally fixed via adhesive bonding or the like to the RR moving ring 514 holding the focusing lens unit 501d on the movable side. A driving magnet 602 and a yoke 603 are fixed to a lens barrel body (not shown, but for example, the rear tube 516) on the stationary side.
The coil 601, the magnet 602, and the yoke 603 constitute a linear motor. When current flows through the coil 601, the linear motor generates a driving force to drive the RR moving ring 514 along the optical axis 505, thus enabling driving the focusing lens unit 501d. 
Meanwhile, in the case of an interchangeable lens barrel, a rearmost focusing lens unit tends to have an increased size and an increased weight. In particular, in the case of a 3-CCD (charge-coupled device) camera system using a 3P (color separation) prism, a long flange back distance and a long back focal distance are required. Accordingly, the above tendency increases.
In addition, an interchangeable lens barrel has a mount at its rear end for coupling with a camera body. A focusing lens unit disposed at the rear most portion of the lens barrel can be located up to an internal space of the mount so as to perform focusing on an infinitely distant object. In the case of the interchangeable lens barrel, the mount has a configuration convex towards the rear (towards the image plane side).
In such a lens barrel, for example, in the lens barrel disclosed in the above-described Japanese Laid-Open Patent Application No. 2002-214504 in which a focusing lens unit is driven by a driving device having a linear motor, a sleeve portion in which a guide bar for the focusing lens unit is fitted is located in a position nearer to the front (the object side) than the center of gravity of the focusing lens unit as viewed in the optical axis direction. Thus, the center of gravity of the focusing lens unit is located outside a space between a front end and a rear end of the sleeve portion. Accordingly, in some cases, due to the unbalance of the focusing lens unit with respect to the center of gravity, the driving device cannot provide satisfactory operation characteristics.
Furthermore, in the case of an interchangeable lens barrel, the rearmost focusing lens unit tends to have an increased size and an increased weight as described above. Accordingly, an actuator for driving the focusing lens unit is required to have a large output, thus resulting in an increase in size. Therefore, in cases where a linear actuator, such as the above-described VCM, is used, a large-sized magnet, yoke, etc., have to be employed.
Furthermore, in the case of an optical system having a high magnification, for example, a 20× magnification, the rearmost focusing lens unit has a large focusing stroke. Accordingly, the plate thickness of a yoke is required to have a large value depending on the large focusing stroke. In this instance, if the plate thickness of the yoke exceeds a given value, it becomes difficult to mass-produce U-shaped yokes. To take measures against this difficulty, it may be possible to widen a U-shaped yoke in a lateral direction thereof and to thin a magnet so that the plate thickness of the yoke can be made small. However, in doing so, an actuator unit composed of the magnet and the yoke increases in size in the lateral direction. As a result, a lens barrel also increases in size.